(a) Field of the Invention
The present invention relates to a VDSL (Very high bit-rate Digital Subscriber Line) system on the basis of a DMT multi-carrier (Discrete MultiTone) line coding method. More specifically, the present invention relates to a VDSL system based on the DMT line coding method using an optimally estimated length of a CP (Cyclic Prefix) sample, and a method for determining the length of the CP sample in the same system.
(b) Description of the Related Art
The VDSL represents a technique for sending high-speed data of 10 through 50 Mbps through a telephone line over a distance (300 through 500 m) that is shorter than that of the ADSL (Asymmetric Digital Subscriber Line). The VDSL is standardized by American T1E1.4, and classified as the CAP (Carrierless Amplitude and Phase modulation) and the DMT line coding methods, and the present invention relates to a VDSL system of the DMT method.
The DMT line coding method divides a total transmission tone into a plurality of tones, and determines formats of transmission signals according to channel environments of the respective tones. In this DMT line coding method, inter-symbol interference generated through channels substantially deteriorates the performance of the whole system. To prevent the influence of the inter-symbol interferences, CPs (cyclic prefixes) are employed.
Usage of the CP samples is to previously repeat and send part of the transmission signals, and accordingly, the transmission signals come to have cyclic features. Deterioration of performance caused by the inter-symbol interference can be prevented when the length of the CP samples is sufficiently long.
FIG. 1 shows a block diagram of a DMT VDSL system applying general DMT line coding thereto.
As shown, the DMT VDSL system comprises a transmitter 100 and a receiver 200.
A scrambler 110 scrambles the signals input to the transmitter 100, an RS (Reed-Solomon) encoder 120 encodes the signals into RS codes which include a slow channel that passes through an interleaver 120 and a fast channel that does not pass through the interleaver 120, and a mapper 130 maps them according to a number of bits allocated to each tone by using the QAM (Quadrature Amplitude Modulation) method.
An IFFT 140 converts the frequency band samples mapped to the respective tones into time band samples, and a CP inserter 150 inserts a CP sample to each time band sample so as to remove the influence of the inter-symbol interference caused by the channel, and outputs them as transmission signals.
The transmission signals into which the CP is inserted and which are output by the transmitter 100 are provided to the receiver 200 through a channel 300.
In the receiver 200, a CP remover 210 removes the CP inserted into the transmission signals received through the channel 300, and outputs clean transmission signals, and an FFT 220 converts them into frequency band signals. An FEQ (Frequency domain EQualizer) 230 compensates for channel attenuation, and a demapper 240 demaps the signals output by the FEQ 230, and outputs result signals, and the result signals are recovered to original signals and are then output by an RS decoder and deinterleaver 250 and a descrambler 260.
FIG. 2 shows CP samples applied to the general DMT line coding method.
FIG. 2(a) shows a format of the transmission signal applying no CP sample thereto. In the case of transmitting the transmission signal through the channel 300, a severe lowering of performance may be generated because of the inter-symbol interference caused by the channel 300.
FIG. 2(b) shows a process for generating a CP sample. The CP is used for reproducing part of the samples of the transmission signals (symbols), in particular the last part, and transmitting the same in advance, and the CP enables the transmission signals to have a cycle.
FIG. 2(c) shows a format of the transmission signal after application of the CP sample through the process of FIG. 2(b). In this instance, a sample number of the transmission signals increases to N+Ncp by adding a sample number Ncp of the CPs to the sample number N which is a sample number of a single symbol.
FIG. 2(d) shows a process for removing CP samples from received sample signals, and extracting original transmission signals. Through this process, clean original transmission signals from which the inter-symbol interferences caused by the channel 300 are removed, are extracted.
The above-noted usage of the CP samples has been widely used in the DMT line coding. For example, U.S. Pat. No. 5,285,474 discloses a method for using CP samples with fixed lengths, and utilizing a complex TEQ (Time domain EQualizer) technique to control the influence of the inter-symbol interference caused by a channel.
Differing from the DMT ADSL system that use the complex TEQ technique because of a very long channel, the VDSL system can obtain sufficient capability not by using the TEQ technique but by using CP samples with appropriate lengths, and hence, it is very important to effectively measure the length of the CP samples and apply it.
That is, as shown in FIG. 2, transmission efficiency is greatly reduced when maximally fixing the length of the CP samples and repeatedly sending the original transmission signals.